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Nat. Hazards Earth Syst. Sci. Discuss., 3, 2537–2564, 2015www.nat-hazards-earth-syst-sci-discuss.net/3/2537/2015/doi:10.5194/nhessd-3-2537-2015© Author(s) 2015. CC Attribution 3.0 License.

This discussion paper is/has been under review for the journal Natural Hazards and EarthSystem Sciences (NHESS). Please refer to the corresponding final paper in NHESS if available.

Movement of the Donglingxin landslide,China, induced by reservoir inundationand rainfallJ. Yu1,2, R. B. Wang1,2, W. Y. Xu1,2, L. Yan1,2, J. C. Zhang1,2, and Q. X. Meng1,2

1Research Institute of Geotechnical Engineering, Hohai University, Nanjing, China2Key Laboratory of the Ministry of Education for Geomechanics and EmbankmentEngineering, Hohai University, Nanjing, China

Received: 22 March 2015 – Accepted: 31 March 2015 – Published: 15 April 2015

Correspondence to: R. B. Wang ([email protected])

Published by Copernicus Publications on behalf of the European Geosciences Union.

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Abstract

With numerous high mountains, deep valleys and turbulent rivers, many hydropowerplants have been constructed in the south-west China. Reservoir bank slopes are verycommon in this area, these slopes are widespread and quite often involved in defor-mation that can result in serious damage and casualties. In case of the Donglingxin5

landslide, for an in-depth study of processes that can trigger these events, the defor-mation characteristics and the failure mechanisms of the slope were performed on adetail scale, based on an intensive monitoring of rainfall events, reservoir level fluctua-tion and groundwater movement. The deformation of the upper part of slope is mainlyinduced by rainfall events, reservoir level fluctuation affects the deformation of the lower10

part of slope. The increase of pore water pressure may result in the failure of slope.The filed investigation suggest that the slope is unstable. Drainages is the only sta-bilization measure which can be implemented, due to very complex geological andgeomorphology condition.

1 Introduction15

In recent years, a large number hydropower plants have been constructed in south-west China, and many landslides are reactivated by initial impoundment of the reser-voirs, so instabilities of reservoir bank slopes have been a challenging issue for hy-dropower projects. Many failure events of reservoir bank slope have been reportedin China (Huang, 2009). In 1961, a reservoir-induced landslide near the Zhaxi dam20

claimed the lives of over 70 workers (Jin and Wang, 1988). In 2003, the Qianjiangpinglandslide killed 24 people and destroyed 346 houses (Wang et al., 2004). The landslide-related direct and indirect economic losses in China costs more than 20 billion Yuanevery year (Bai et al., 2014).

Researchers in China studying the deformation characteristics and triggering mech-25

anisms of landslides (Qi, 2006) found that reservoir level fluctuation and rainfall cause

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deformation leading to landslides (Wang et al., 2008).Infiltration of rainfall acceleratesthe deformation of the landslide and deteriorates its stability (Wang et al., 2012).

Sanbanxi procject is located on the lower reach of the Qingshui River (Fig. 1), themain stream of the Yuanjiang River. It is the sole hydropower plants with multi-yearregulating ability on the Yuanjiang river. It has a concrete face rockfill dam with max-5

imum dam height of 185.5 m and a total installed capacity of 1000 MW (4×250 MW).The reservoir has a normal pool level 475 m and a total capacity of 3.75 billionm3. Thetotal area of the reservoir is 79.56 km3, and the backwater of the reservoir is 120 kmlong. The Donglingxin landslide is located on the south bank of the Qingshui River,southeast of Liuchuan, Jianhe Country, Guizhou Province. It’s is 80 km upstream of10

the Sanbanxi dam (Fig. 1), on which there is a village of dozens of inhabitants. Witha volume of 20.7 millionm3, it is the largest ancient slide in the reservoir. The slide wasreactivated by the first impoundment of the reservoir in July 2007. Due to the narrowvalley at the slide location, the supports and protections measure is difficult to be used.The Liuchuan town in the immediate vicinity will be destroyed, once the slide fails.15

Therefore, the study of its deformation characteristics and the failure mechanisms issignificant for assuring the safety of reservoir and dam.

In this paper, we discuss the deformation characteristics and triggering mechanismof the Donglingxin landslide through geological investigation and comprehensive anal-ysis of the 2 year observation data. The deformation tendency was also assessed.20

2 Description of the Donglingxin landslide

The Donglingxin landslide is a reservoir bank slope, its natural slope surface mainlytends toward N 35◦W. There are two gullies on the edge of landslide. Geological sur-vey indicated that the Donglingxin landslide is a large-scale ancient landslide, whichis an accumulation of several consequent slides along the bank slope, several sec-25

ondary slumps occurred in its front, most of the landslide area is underlain by coarsesilty clay and stone, relaxed and broken rock mass was found only at the foot of the

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excavation slope and the central ridge. The landslide can be divided into two zones,Fig. 2 shows the topography and zoning of the Donglingxin landslide (Google earth),(i) zone 1 extends along the Qingshui River with approximately 280 m in height and310 km2 in area, it is 800 m wide and lies between the riverbed and elevation 700 m,and the landform looks like a round-backed armchair (ii) zone 2 is located in the upper5

southern region of zone 1 with 120–220 m in height and 68 km2 in area it is 200 m wideand lies between elevation 600 and 825 m. Borehole data indicate that the landslideis generally between 18 and 96 m thick, the thickest being about 140 m. The landslidevolume is about 20 millionm3. The slope of zone 1 ranges from 22◦ in the upper partto 40◦ in the lower part. The slope of zone 2 ranges from 17 to 35◦. Houses are built10

at elevations of 670–702 m. Figure 3 shows the local village and rice paddies at anelevation of 692 m.

The landslide is located in a sub-humid subtropical climate zone with a mean an-nual precipitation of 1280.5 mm. Most of precipitation occurs in the months from Aprilto August with a maxmum daily precipitation up to 133.3 mm. The lowest water level15

of the Sanbanxi Reservoir is about 425 m, while the highest water level of the reser-voir after inundation is 472 m. Groundwater in slope is pore phreatic in Quaternaryand bedrock fissure water. The precipitation recharges groundwater within the land-slide. There are two drainage paths: (1) through fracture network downwards into theunderlying bedrock, (2) discharge into the gully directly.20

3 Landslide geology

The stratum outcropped in this region are Banxi group of Algonkian system and Qua-ternary. The lithology at the landslide is illustrated in three profiles presented in Fig. 5along the sections marked by I-I′, II-II′, and III-III′ in Fig. 4. Stratigraphic geo-materialsdiffer in compositions and characteristics as described in detail below.25

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1. Member 1 of Qingshui formation of Banxi Group of Algonkian system (P 1tbq): mainly

consisting of blastobedding tuffaceous sandstone, tuffaceous silty slate and blas-tobedding tuff.

2. Quaternary, including eluvium and diluvium (Qedl4 ), accumulated layer landslide

(Qdel4 ) and human accumulation layer (Qs

4)5

Eluvium and diluvium (Qedl4 ) is comprised of silt clay, with mixture of broken stones,

it is found in the slope mostly behind the village and on both sides of the ridgewith thickness of 1–5 m.

Accumulated layer landslide (Qdel4 ) is composed of four types of geological mate-

rials. It is 18–96 m thick. The stratigraphic physical and mechanical properties are10

given in Table 1 (provided by HydroChina Zhongnan Engineering).

i. Coarse-grained silty clay with mixture of strongly weathered broken stonesare distributed on the surface of the landslide except at the lower part of zone1 and east of zone 2. The content rate of rock is 20–40 %. The thickness isgenerally 5–12 m with some places at the upper part of the deposit over 20 m15

thick.

ii. Strongly weathered broken stones and gravel with mixture of silty clay wasobserved in the middle of the landslide, and covered with layer i. The contentrate of rock in this strata is high. The thickness of this strata varies greatly,from 6.3–12 m in the west and north to 51.2–79.1 m in the central and south-20

ern parts of the landslide.

iii. Cataclastic rock mass comprised of blastobedding tuffaceous sandstone,tuffaceous silty slate and blastobedding tuff, is distributed mainly at the lowerpart of the landslide. This strata has moderate or strong weathering, withgood rock mass integrity. The thickness is over 120 m.25

iv. The sliding zone mainly consists of yellow brown and dust color silty clay withthickness of 1.3–11.3 m, a small amount of gravel distributed in the front.

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Human accumulation layer (Qs4) is gravel, about 5 m thick, laid during the

highway excavations.

4 Deformation features and stabilization measures

4.1 Deformation features

The first impoundment of the Sanbanxi reservoir began in January 2006. At the end5

of July 2007 the reservoir level reached 472, 3 m below the normal storage level. Fieldinvestigations showed that cracks appeared at the concrete foundation of house (Fig. 6)and the west boundary of zone 1 at elevations of 665–700 m where the head of gully atthe end of July 2007. The deformation tendency of the surface of landslide aggravatedin 2008. This newly developed deformation is described below.10

1. In the eastern gully, where the landslide is covered by quaternary overburden,a few cracks 1–5 cm wide and about 2 m long were observed along N 10◦W–N 20◦ E.

2. In the rice paddies at elevation 692 m, along N 10◦W–N 5◦ E, a tension crack wasobserved; it was 15–20 cm wide, more than 2 m long, and 10–30 cm in visible15

depth (Fig. 6).

3. The landslide experienced prolonged periods of intense rainfall, muddy water wasobserved coming out of the toe of zone 1 (Fig. 7).

The field investigation suggested that there are two sliding surface in the slope (Fig. 5).One is shallow located within the lower accumulated layer landslide, which has a ro-20

tational slide mode. The other is deep through the sliding zone. It is a translationalslide, and the slope may appear bedding landslide. The deformation characteristicsshow: the landslide belongs to thrust load caused landslide, the landform and material

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buildup is geological condition of landslide, abundant rainfall and reservoir fluctuationare primary induction factors of landslide.

4.2 Stabilization measures

Due to the narrow valley at the landslide location, the supports and protections mea-sure is difficult to be used. Many researches (Fleming et al., 1989; Iverson et al., 1997)5

suggested that, the increase of pore water pressure caused the decrease of shearstrength, therefore, drainage is only stabilization measure which can be used.

Figure 8 shows a drainage tunnel at 500 m elevation, excavated through the bottomof the landslide. To reduce the saturation level at the sliding zone and increase landslidestability, drainage holes were drilled in the tunnel for draining the groundwater around10

the sliding zone, and the bedrock fissure water. The drainage tunnel construction beganin March 2010. Figure 9a shows the drainage tunnel under construction, and the waterdraining out of the tunnel wall can be seen in Fig. 9b. The main tunnel was completedin May 2011.

4.3 Monitoring system15

A large number of monitoring instruments were installed to enable real-time recordingof the slope deformation and analysis of the deformation tendencies. Two monitoringmethods – geodetic detection and borehole monitoring – were used to establish thevectors of the dome displacements and to measure the groundwater level of the slope,respectively.20

For geodetic detection, three horizontal displacement datum points and three ver-tical displacement datum points made of concrete were constructed, as well as nineobservation points at the surface of the slope (Fig. 8). In October 2009, observationdata of the slope deformation were obtained. Additionally, eight boreholes were drilledfor measuring groundwater level.25

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5 Monitoring results and analysis

5.1 Groundwater

The data of groundwater level from the eight boreholes and the reservoir, rainfall dataare plotted in Fig. 10. Looking at the groundwater level of boreholes ZK11 and ZK14located in the lower part of landslide, four fluctuations can be identified.5

1. From November 2009 to February 2010 the groundwater level at ZK11 and ZK14dropped from 452.32 and 475.83 to 428.19 and 455.70 m, respectively. In thisperiod, rainfall was low and the reservoir drainage system had been operating forthree months; hence, these decreases in groundwater level are related to reser-voir drainage.10

2. From May 2010 to July 2010, the groundwater level at ZK11 and ZK14 rose by36.17 and 8.89 m, respectively, because of increased reservoir water level. Thiswas a period of heavy rainfall in the region, which also affected the groundwaterlevel of boreholes ZK11 and ZK14.

3. From December 2010 to March 2011, the groundwater level at ZK11 and ZK1415

dropped from 466.14 and 470.37 to 434.67 and 455.23 m, respectively. The de-creases in groundwater during this period were induced by reservoir drainage.

4. From April 2011 to July 2011, the groundwater level at ZK11 and ZK14 increasedfrom 428.52 and 453.48 to 454.57 and 461.08 m, respectively. The influence ofthe heavy rainfall on the groundwater level is more noticeable, because although20

the water level of the reservoir decreased during this period, the groundwater levelincreased.

At the upper part of the landslide (borehole ZK2; Fig. 10), three distinct phases can beidentified in the groundwater level curve of KZ2, with May 2010 and May 2011 beingthe demarcation points.25

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1. Before May 2010, the groundwater level at ZK2 gradually dropped from 645.49 to617.91 m. In this period, low rainfall levels led to a decrease in the groundwaterlevel.

2. From May 2010 to May 2011, the groundwater level at ZK2 increased from 617.91to 647.88 m following heavy rainfall, and then fell to 629.09 m.5

3. Since May 2010, the groundwater level at ZK2 increased from 629.09 to 652.21 mbecause of heavy rainfall, and then fell to 638.40 m.

Overall, as shown in Fig. 10, changes in the reservoir levels mainly affected the ground-water level at the lower part of the landslide, showing s positive correlation between thetwo levels. The permeability of the lower part of the landslide is good. In the upper part10

of the landslide, the groundwater accumulates because of rainfall; therefore, the per-meability of the upper part is poor.

5.2 Deformation and triggering events

Figure 11 shows the monitoring results of displacement over the 2 year recording pe-riod between October 2009 and September 2011. The rainfall near the landslide and15

the reservoir water level are also shown. Taking, for example, the deformation trend ofthe landside, three stages can be identified.

1. From October 2009 to May 2010 the reservoir water level dropped from 466 to426 m. The displacements at DLXG02, DLXG03, DLXG05, and DLXG06 (Fig. 11)increased rapidly because of reservoir drainage. The biggest deformation oc-20

curred in the lower part of the landslide, after January 2010. There is no significantchange in the reservoir water level during this time and the displacements at thesefour observational points decrease slowly.

2. From May 2010 to March 2011 the reservoir water level rose from 426 to 466 mat first and then dropped from 470 to 428 m. There were many fluctuations in the25

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reservoir water level during this period and great increases in the displacementsof DLXG02, DLXG03, DLXG05, and DLXG06 from May 2010 to July 2010. Thebiggest displacement increment was about 12.08 mm. From July 2010 to Septem-ber 2010 the reservoir water level gradually dropped by 6 m, but the deformationof these four observational points decreased by 5.77, 4.10, 2.17, and 4.22 mm, re-5

spectively. There were two increases in the displacements of DLXG01, DLXG08,and DLXG09 located in the upper part of the landslide; one occurred after therainfall events in June 2010 and the other occurred after the rainfall events inSeptember 2010.

3. After March 2011 the displacements of DLXG02, DLXG03, DLXG05, and10

DLXG06, induced by fluctuations of the reservoir water, increased; the displace-ments of DLXG01, DLXG08, and DLXG09 increased sharply after rainfall events.

Before this period, the displacement of DLXG04 increased slowly; after April 2011,once the main drainage tunnel was completed, the displacement of DLXG04 decreasedby 7.32 mm in May 2011 and rapidly increased after May 2011.15

5.3 Movement tendency

With geological investigation and monitored data, it is found that movement of theDonglingxin landslide can be divided into two phases:

1. Creep deformation: before April 2010, as shown in Fig. 11. The rate of this creepmovement of landslide was increased slowly and gradually with a movement rate20

of 0.72–2.12 mmmonth−1.

2. Instantaneous deformation: roughly since April 2010. The execution of work andinundation were the two principal triggers which activated the landslide, further-more, rainfall induced the shallow rotational slide.

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6 Stability analysis

The movement of the Donglingxin landslide can be attributed to the strength drop of thesliding zone caused by the increase of pore water pressure, the shear strength of thesliding zone, characterized by the cohesion c and the friction angle ϕ are given in Ta-ble 1 (provided by HydroChina Zhongnan Engineering). The shear strength decreased5

from 23 kPa and 29.4◦ for the natural state to 20 kPa and 28◦ for saturated state.Three representative engineering profiles: preflies I-I′, II-II′, and III-III′ (Fig. 5) were

selected for stability analysis by modified Janbu method under three different condi-tions: a natural state, a rainfall and an inundation with rainfall. The safety factors of twosliding surfaces were calculated, as given in Table 2.10

From Table 2, it is interesting to find that, rainfall had a great negative influence onthe stability of the shallow sliding surface, but a minor influence on the stability of thedeep sliding surface. Inundation was the opposite, had a great negative influence onthe stability of the deep sliding surface.

7 Disaster prediction15

The consequence of numerical simulation by YADE (Fig. 12) shows that once theDonglingxin landslide fails, stones and soil with the total debris volume reaching sev-eral millions and waves surging to a height of about 34 m. the upriver water level willbe rised to 478–490 m by river blocking up in the Qingshui river, the affected area isabout 23 km range. Consequently, the Liuchuan town which is 1.5 km away from the20

landslide with 22 434 inhabitants will be destoryed by surge. 14 km away from the land-slide, a small hydropower station with 486 m in elevation will be submerged. The Jianhecounty town with 44 057 inhabitants and 476.6 m in elevation is 20 km away from thelandslide, it will also be submerged.

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8 Conclusions

Based on monitoring measurements over a period of about 2 years and stability as-sessment we conclude the following.

1. The geological investigation indicated that the Donglingxin landslide is a large-scale ancient landslide, which is an accumulation of several consequent slides5

along the bank slope, the slope may appear bedding landslide, and the deforma-tion characteristics shows that the landslide belongs to thrust load caused land-slide.

2. Because the accumulated layer landslide is rich in broken stone and gravel, it hasgood permeability, the rainfall water and reservoir water are prone to permeate10

through this stratum into the depth of the slope. Thus, they have effect on thestrength reduction of the landslide.

3. Due to the narrow valley at the landslide location, the supports and protectionsmeasure is difficult to be used, but the movement near the drainage tunnel maybe more active after a few months of drainage.15

4. Movement of the lower part of the Donglingxin landslide corresponds to waterlevel changes in the Sanbanxi reservoir. The changes are more evident when thereservoir water level rises. Movement of the upper part of Donglingxin landslidecorresponds to rainfall events. The movement is greater during and after the wetseason.20

5. The groundwater level at the lower part of the Donglingxin landslide correlateswell with the reservoir water level. The groundwater at the upper part of theDonglingxin landslide corresponds to rainfall events.

6. Stability assessment indicate that the rainfall had a great negative influence onthe stability of the shallow sliding surface, but a minor influence on the stability of25

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the deep sliding surface. The inundation was the opposite, had a great negativeinfluence on the stability of the deep sliding surface.

7. Once the Donglingxin landslide occurrs, the affected area which will be sub-merged is about 23 km range.

Acknowledgements. This work is financially supported by National Key Technology Research5

and Development Program (Grant No. 2013BAB06B01), and the National Natural ScienceFoundation of China (Grant no. 51209075, 51479049,51409082), and the Fundamental Re-search Funds for the Central Universities (Grant no. 2014B17714).

References

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occurrence with rainfall: a case study of Wudu County, China, Arabian J. Geosci., 7, 1277–1285, 2014.

Fleming, R. W., Ellen, S. D., and Algus, M. A.: Transformation of dilative and contractive land-slide debris into debris flows-an example from Marin County, California, Eng. Geol., 27, 201–223, doi:10.1016/0013-7952(89)90034-3, 1989.15

Huang, R. Q.: Some catastrophic landslides since the twentieth century in the southwest ofChina, Landslides, 6, 69–81, 2009.

Iverson, R. M., Reid, M. E., and LaHusen, R.: Debris-flow mobilization from landslides, Annu.Rev. Earth Pl. Sc., 25, 85–138, 1997.

Jin, D. L. and Wang, G. F.: Tangyanguang Landslide in Zhaxi Reservoir, Hunan Province, China,20

Typical Landslides Occurred in China, Science press, Beijing, 301–307, 1988 (in Chinese).Qi, S., Yan, F., Wang, S., and Xu, R.: Characteristics, mechanism and development tendency

of deformation of Maoping landslide after commission of Geheyan reservoir on the QingjiangRiver, Hubei Province, China, Eng. Geol., 86, 37–57, 2006.

Wang, F., Zhang, Y., Huo, Z., Matsumoto, T., and Huang, B.-L.: The July 14, 2003 Qianjiangping25

landslide, Three Gorges Reservoir, China, Landslides, 1, 157–162, 2004.Wang, F., Zhang, Y., Huo, Z., Peng, X., Araiba, K., and Wang, G.: Movement of the Shup-

ing landslide in the first four years after the initial impoundment of the Three Gorges DamReservoir, China, Landslides, 6, 321–329, 2008.

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Wang, R., Xu, W., and Zhang, J.: Comprehensive assessment and sliding mechanism analysisof Zhenggang Landslide, Southwestern China, Disaster Advances, 5, 327–331, 2012.

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Table 1. Stratigraphic physical and mechanical parameters.

Triaxial shear strengthStrata c (kPa) ϕ (◦) Density (gcm−3)

Natural Saturated Natural Saturated Natural Saturated

Qdel4 i. Coarse-grained silty clay with

mixture of broken stones20 18 26 24 2.0 2.1

ii. Broken stones and gravel withmixture of silty clay

20 15 35 30 2.15 2.22

iv. Sliding zone 23 20 29.4 28 2.03 2.08

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Table 2. Results of stability analysis with modified Janbu method.

Profile Sliding surface Natural rainfall Inundation with rainfall

I-I′ shallow 1.133 1.085 1.014deep 1.155 1.105 1.023

II-II′ shallow 1.140 1.077 0.995deep 1.170 1.132 1.035

III-III′ shallow 1.172 1.127 1.028deep 1.209 1.118 1.045

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Figure 1. Location map of the Sanbanxi dam and the Donglingxin landslide, China.

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Figure 2. The Donglingxin landslide facing the Qingshui River (taken form Google earth).

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Fig. 3 Photo of local village and farmland

Fig .4 Topography and monitoring locations of the Donglingxin landslide

Figure 3. Photo of local village and farmland.

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Figure 4. Topography and monitoring locations of the Donglingxin landslide.

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Figure 5. Geological profile of the landslide deposit.

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Fig. 5 Geological profile of the landslide deposit

Fig. 6 Photo of cracks in farmland at elevation 692 m Figure 6. Photo of cracks in farmland at elevation 692 m.

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Fig. 7 The phenomenon of muddy water coming from the landslide

Figure 7. The phenomenon of muddy water coming from the landslide.

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Figure 8. Location of the observation points in the Donglingxin landslide.

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Fig. 8 Location of the observation points in the Donglingxin landslide

Fig. 9 Construction of drainage tunnel EL 500

Fig. 10 Monitoring results of groundwater levels at boreholes ZK1–ZK5, ZK11, ZK13, and ZK14 for the period October 2009 to

September 2011 in the Donglingxin landslide. Rainfall for this period and water level in the Sanbanxi reservoir are also shown.

(a) (b)

Figure 9. Construction of drainage tunnel EL 500.

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Figure 10. Monitoring results of groundwater levels at boreholes ZK1–ZK5, ZK11, ZK13, andZK14 for the period October 2009 to September 2011 in the Donglingxin landslide. Rainfall forthis period and water level in the Sanbanxi reservoir are also shown.

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Figure 11. Monitoring results of displacements for the period from October 2009 to Septem-ber 2011 in the Donglingxin landslide. Rainfall for this period and the water level in the SanbanxiReservoir are also shown.

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Figure 12. The consequence of numerical simulation by YADE.

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